Such a process is described in EP-A-168892 of applicant in 1986. According to this publication the endothermic reaction is preferably carried out in a fixed bed situated in at least one pipe in which a temperature of between 800 and 950° C. is maintained by routing at least part of the hot product gas from the partial oxidation along the pipe(s). According to this publication the combined partial oxidation and endothermic production of synthesis gas result in a better yield of synthesis gas, an increased H2/CO ratio, a lower usage of oxygen per M3 of syngas product obtained and a lower capital cost of the plant for the production of CO and H2-containing gas mixtures. An example of the process as described in EP-A-168892 is described in EP-A-326662.
EP-A-171786 discloses a similar process as EP-A-168892. The difference is that the product gas having the elevated temperature is not prepared by means of partial oxidation of natural gas but in a conventional reformer furnace wherein burners provide the required heat. This first process gas is then cooled in a so-called Enhanced Heat Transfer Reformer (EHTR) by routing this gas along the exterior of tubular pipes of the EHTR. EHTR reactors and the like are generally referred to as Convective Steam Reformer (CSR). These pipes contain a fixed bed of catalyst for performing an endothermic reforming reaction using a second part of the natural gas feed. The mixture of carbon monoxide and hydrogen as obtained within the tubes could be considered to be the second product gas. The product gas as obtained in the conventional reformer contains approximately 33% steam.
U.S. Pat. No. 6,224,789 discloses a similar process as described above except that the product gas having the elevated temperature is prepared from natural gas in a so-called Autothermal Reformer (ATR) in the presence of a Ni-containing catalyst and steam. The hot product gas is then contacted with the exterior of the reactor tubes of an EHTR like reactor.
The tubes of a Convective Steam Reformer are typically made from metal alloys comprised substantially of iron. Iron containing alloys are preferred because of their mechanical strength in combination with their relative low cost. Furthermore, usage of these alloys makes it possible to manufacture the complicated tube structures of such an apparatus. A disadvantage of the above apparatus is that in use, coke will form on the exterior surface of the tubes because part of the carbon monoxide reacts to form carbon and carbon dioxide. Furthermore, part of the surface will erode, eventually resulting in an unacceptably low mechanical integrity of the tubes. These effects are especially significant when the amount of steam in the hot gas is below 50 vol %. Such a hot CO and H2 containing gas, is for example, obtained when performing a partial oxidation of natural gas, refinery gas, methane and the like in the absence of added steam as described in WO-A-96/39354. There is thus a need for an improved process if one intends to operate a partial oxidation and a reforming process in combination, as for example described in EP-A-168892 or in EP-A-326662.
The object of the present invention is to provide a process having the advantages of the process of EP-A-168892 or EP-A-326662 wherein less or no coke formation and/or erosion on the exterior of the reactor tubes occurs.